The present invention relates to an optical modulator, and particularly to a traveling wave optical modulator and an optical communication light source in optical communication systems.
A faster light source for optical communication has been demanded for an optical communication system. Currently, a communication system with a transmitting speed of 10 Gbit/s for each channel has been already put into practical use, and a system with a transmitting speed of 40 Gbit/s for each channel has been developed for practical use. As an optical communication light source used in such a high-speed optical communication system, an optical modulator has been adopted.
As an optical modulator, two kinds of an electroabsorption modulator and a Mach-Zehnder type modulator have been used. For example, as technique for achieving a wide-band modulator as an electroabsorption modulator, JP-A-2000-258739 discloses that a modulator is divided to reduce a capacitance thereof, thereby achieving a wide-band modulator with a high extinction ratio.
Further, JP-A-H11-133366 discloses that a traveling wave electrode is used as an electrode, a thickness of a non-doped layer is reduced, and a modulator length is shortened, thereby achieving a wide band, low return loss of an input electrical signal, and a reduced amplitude of a driving voltage.
Moreover, as for a Mach-Zehnder type modulator as well, JP-A-H07-98442 discloses that an impedance conversion circuit is used for reducing amplitude of a driving voltage to maintain a wide band and low return loss. At this moment, impedance of a traveling wave electrode, which constitutes the Mach-Zehnder type modulator, is set lower for reducing a driving voltage, and an impedance matching circuit is used for impedance matching with a transmission line disposed outside an optical modulator having high characteristic impedance. At this moment, an inductance, a capacitance, and a resistance of a lumped element are used for the configuration, and a wide band and convenience of the configuration are not compatible with each other.
When a driving voltage is reduced, a capacitance of a modulator is increased or impedance is lowered. Thus, reflection occurs on an electrical signal inputted to an optical modulator, a band is reduced, or a deviation of response occurs inside a band. Besides, when a capacitance is reduced or a modulator length is shortened for impedance matching, an extinction ratio is deteriorated. The above problem can be solved by increasing a thickness of a multiple-quantum-well layer. However, when strong laser light is emitted to the modulator, high-frequency characteristics are deteriorated as follows: when a modulator length is short, a density of photo carriers is increased on an undoped layer including a multiple-quantum well and the adjacent region, an electric field applied from the outside is reduced by screening, response to electricity is deteriorated, and a band is reduced.
Additionally, when a modulator is divided into a plurality to widen a band, a band is determined by a capacitance, a resister of a divided modulator, and a terminal resister of a divided modulator and a load resister of modulator driver circuits. In order to widen a band, it is necessary to increase the number of divided modulators. At this moment, the problem is loss of light at a part where two modulators are connected. Further, when the number of divided modulators is reduced, an optical modulator needs to be connected by an electric line having an appropriate inductance for impedance matching. However, since the number of divided modulators is reduced, it is necessary to increase a length of the electric line, resulting in a larger size of the element.
Moreover, when impedance mismatch is matched by an impedance conversion circuit and when impedance lowered for reducing a driving voltage amplitude is matched by a driver circuit, it is not possible to achieve constant response over a wide band.
The following measures are taken to solve the above problems. As shown in FIG. 1, an optical modulator is prepared which includes:
an optical waveguide 1 for propagating light inputted from a laser;
an optical modulator part 2 for varying the intensity of light by applying voltage;
a modulator driver circuit 3 for producing or amplifying an electrical signal to be communicated;
a terminal resister 4 disposed on an end of the optical modulator part that is opposite from the driver circuit;
a transmission line 5 for transmitting an electrical signal to the optical modulator part, the transmission line being provided between the driver circuit and the optical modulator part; and
a transmission line 6 between the optical modulator part and the terminal resister.
Assuming that lines (straight lines 7 and 8) perpendicular to the direction of the optical waveguide are provided on both ends of the optical modulator in the direction of the optical waveguide, an electrical signal, which is inputted from the driver circuit to the optical modulator part, strides over (or intersects) the straight line 7 or 8. Further, at least one of the transmission lines 5 and 6 is set such that at least part of characteristic impedance thereof is larger than the output impedance of the driver circuit 3 and it has a suitable length. With this configuration, a driving voltage is reduced and an optical output is improved. Therefore, impedance matching can be carried out on a traveling wave electrode of the optical modulator part, which decreases in characteristic impedance. Thus, it is possible to maintain a wide band of an input electrical signal, reduce a deviation inside a band, and decrease electrical reflection. For example, when the modulator part has impedance of 21xcexa9 and a modulator length of 150 xcexcm, as disclosed in JP-A-H11-133366, a modulator driver circuit system is constituted by a driver circuit having a driver circuit output impedance of 50xcexa9 and a terminal resistor of 50xcexa9.
When description is made using the same members as FIG. 1, in order to design transmission lines (45 and 46 in FIG. 4), which correspond to the transmission lines 5 and 6, at characteristic impedance of 50xcexa9 or an approximate value, the transmission lines 45 and 46 are reduced in width while ratios of widths of the transmission lines 45 and 46 and distances between a ground 53 and the transmission lines 45 and 46 are maintained at values corresponding to a thickness and a dielectric coefficient of a substrate. FIG. 5 shows return loss (S11) and high frequency response (S21) at this moment.
In this case, it is assumed that a data electrical signal of 40 Gbit/s is inputted and used. No problem occurs on the high frequency response (S21) but the return loss (S11) has a large reflectivity of xe2x88x9210 dB at 20 GHz, an electrical signal inputted to the modulator may return to the driver circuit and destroy the driver circuit. Further, since reflection cannot be eliminated on an output end and so on of the driver circuit, multiple reflection varies a modulator driving voltage, thereby deforming an optical waveform.
Meanwhile, FIG. 1 shows an example of the configuration in which the transmission lines 5 and 6 having characteristic impedance of 111xcexa9 are disposed at the front and rear of the optical modulator part 2 having characteristic impedance of 21xcexa9. At this moment, the impedance of the transmittance lines 5 and 6 can be increased by the following configuration. Ratios of widths of the transmission lines 5 and 6 and distances between the ground 13 and the transmission lines 5 and 6 are reduced, and metal or a layer doped with a high electrical conductivity is placed sufficiently apart under the transmission lines. In this example, a conducting layer right under the transmission lines 5 and 6 is a metallic jig for fixing a semiconductor substrate.
The cross-section of this part is shown in FIG. 3. Reference numeral 31 denotes an electrode serving as a transmission line, reference numeral 32 denotes a polyimide material having a low dielectric coefficient, reference numeral 33 denotes an anti-insulated substrate, and reference numeral 34 denotes an electrode corresponding to the ground 13. In the case where a data signal of 40 Gbit/s is inputted to the optical modulator in the driver circuit configuration similar to that using the 50-xcexa9 transmission lines, high frequency response is not deteriorated and a reflectivity at 20 GHz can be reduced by about 8 dB.
In such improvement in reflectivity, a high impedance line acts as an impedance conversion circuit. As for the optical modulator part, in the case of an electroabsorption modulator using a multi-quantum well as the optical modulating part, conventionally, as shown in FIG. 7, when intensity of light inputted to the optical modulator is at or larger than a fixed value, high frequency response deteriorates. In the case of such an element, an input limit of the optical modulator is about 9 dBm.
It is considered that such deterioration of high frequency response is resulted from two causes. One is that carriers are accumulated on an MQW layer and light cannot be sufficiently absorbed due to a high frequency electrical signal. The other cause is that carriers are accumulated on the MQW layer and the adjacent undoped layer so as to reduce an electric field, resulting in a lower absorption coefficient of the MQW, and more photo carriers are accumulated due to a longer escape time of photo carriers, thereby the high frequency response rapidly deteriorating.
To solve the above problem, since the MQW layer is reduced in thickness, when a strong electric field of several tens kV/cm or more is applied, an electron or a positive hole once excited from a quantum well to continuous state is drawn to a layer disposed outside a quantum well without being drawn to the quantum well again. Thus, it is possible to shorten running time of carriers, restrict accumulation of photo carriers, and prevent high frequency response even when an intensity of input light is large.
Further, it is possible to reduce a density of photo carriers appearing due to small superimposition of fields of light, which is inputted to the MQW layer and propagated in the optical modulator part. Moreover, it is possible to reduce a density of photo carriers accumulated on the MQW layer and the adjacent undoped layer and suppress a reduction in an electric field that is caused by the accumulation of photo carriers, thereby preventing deterioration of high frequency response. At this moment, it is necessary to increase a modulator length to avoid deterioration of an extinction ratio. Deterioration of high frequency characteristics can be prevented by adjusting an impedance value and a length of a high impedance line at the front and rear of the optical modulator part.